The invention relates to a process for recoating a deactivated catalyst coating in a catalyst for removing harmful constituents from the exhaust gas of an internal combustion engine, particularly in a motor vehicle, by applying noble metals to the catalyst. The invention further relates to an apparatus for carrying out the process described.
A process of the generic type is disclosed in German Patent Document DE-B 1 100 600.
Catalysts have a limited life so that after some time it may be found in an exhaust gas test in a workshop that the emission values for a vehicle equipped with a catalyst are too high.
One cause of these unacceptably high emission values can be inactivation of the catalyst coating. Apart from eliminating the actual causes, for example a damaged ignition system, the catalyst also has to be replaced subsequently. This is extremely laborious and therefore costly, particularly in the case of systems in which the catalytic converter is welded to the adapter pipes.
The point should be made that hereinafter the term "catalyst" refers not only to the monoliths which carry out the actual exhaust gas purification process, but to the entire catalyst unit with the sometimes firmly fastened adapter pipes.
The above-mentioned German Patent Document DE-B 1 100 600 discloses a process for activating catalysts comprising metals of the platinum group. In this process, small amounts of dust which has already been used for catalytic purposes and comprises metals of the platinum group are applied to the surface of catalyst bodies of the metals of the platinum group.
However, a disadvantage of the process disclosed in said publication is that the entire catalyst has to be disassembled, since the platinum black formed has to be cleaned off from the catalyst using a soft brush and the catalyst body then has to be tapped to knock off adhering material. Subsequently, activated dust of metals of the platinum group is applied by powdering, distribution with brushes, blowing on or in another suitable manner, so that the process described is very laborious and therefore costly since, as already mentioned, the catalyst has to be disassembled for this purpose.
It is therefore an object of the present invention to provide a process for recoating a deactivated catalyst coating of a catalyst, by means of which the recoating of the catalyst can be carried out in a simple and therefore inexpensive way.
It is also an object of the invention to provide an apparatus for carrying out the said process.
These objects are achieved according to preferred embodiments of the invention by providing an arrangement wherein a coating and/or noble metal compounds are sprayed by means of an aerosol into the catalyst, wherein the noble metal compounds are then brought into their elemental state by means of a reducing medium flowing through the catalyst, and wherein the coating applied is dried by heat treatment.
The spraying of a coating and/or of noble metal compounds by means of an aerosol into the catalyst can make the laborious, time-consuming and therefore expensive disassembly of the catalyst or the catalyst unit unnecessary, since the catalyst unit only has to be removed from the vehicle, but can remain assembled as a unit.
If the noble metal compounds introduced into the catalyst by means of the aerosol are subsequently reduced by a suitable medium and thus brought into their elemental state, this results in pure noble metals which can be dried by a suitable heat treatment or calcination, so that a suitable recoating of a deactivated catalyst is realized.
The noble metal compounds can comprise platinum, rhodium or palladium.
An apparatus for carrying out the process includes a first line for a carrier gas, a second line for the coating and/or the noble metal compounds, with the first line being arranged, at least partly in a region before the catalyst, within the second line and the end of the first line nearest the catalyst being provided with an atomization device, and the second line opening out into the catalyst, a third line provided after the catalyst, which third line opens into a separator from which the first line and the second line lead away, and devices for generating pressure differences provided in at least one of the lines.
By means of the apparatus described, an aerosol can be produced in a simple way, with the aerosol being produced only in a region before the catalyst or in the adapter pipe of the catalyst and there being arranged shortly after the catalyst a separator in which the aerosol is separated into its liquid and gaseous constituents so as to ensure that coating or recoating with noble metal occurs virtually only within the catalyst and, in contrast, coating in the feed and discharge lines does not occur since a carrier gas, e.g., air or an inert or reduced gas, and coating or noble metal compounds are conveyed separately or flow through these lines.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.